LINCOLN, Calif. – Firefighters are letting a burning propane rail tanker burn itself out after determining the rail car held much less propane than previously thought, a fire official said late Wednesday.

Firefighters had spent most of Wednesday preparing for a bold attempt to drain a burning propane rail tanker to head off a catastrophic explosion in a Northern California town, where thousands of people have been evacuated from their homes.

But fire crews changed tactics late Wednesday, instead working to fill the tank with water and foam as the remaining propane burned off, Lincoln Fire Chief Dave Whitt said

Crews planned to start draining the propane into a newly dug retaining hole at around 5 p.m. They delayed the process and then decided on the new strategy after determining that the tank held much less propane that they had thought.

Whitt said during a 10 p.m. briefing that he expected the new procedure to take up to two hours before the remaining vapor was out of the tank.

"The tank is pretty much empty," he said.

Officials had worked throughout the day Wednesday in trying to head off the potential failure of the 29,000-gallon tank. A buildup of heat could lead to an explosion that Whitt compared to a "small thermal nuclear bomb" that produces a fireball several hundred yards wide.

An explosion also could throw metal shards up to a mile away. Officials on Tuesday ordered mandatory evacuations within a one-mile radius.

Between 4,000 and 5,000 homes in the city of 40,000 people were evacuated, and more than 6,000 students were missing their first days of classes, with district schools ordered closed until Monday.

"The good news is....the problem has gone away," Witt said late Wednesday. "By morningtime we should be golden," he said.

Officials were hoping to lift the mandatory evacuation around midnight, he said.

So far, the burning propane hasn't had a significant impact on the area's air quality, Placer County air pollution control officer Tom Christofk said in a statement.

It was unclear how the tanker caught fire. It was burning at the Northern Propane Energy yard in Lincoln, about 30 miles northeast of the state capital. It was surrounded by trucks, other rail cars and storage tanks containing at least 170,000 gallons of additional propane that Whitt said were at risk as the fire burned. A gas pipeline also runs through the area.

One worker at the rail yard was injured in the initial fire and suffered flash burns but has been released from a hospital.

A similar fire in 1973 in the Arizona town of Kingman killed 11 firefighters and a gas company worker when a rail car carrying a propane tank exploded. The resulting fireball injured more than 100 others and showered the surrounding area with shrapnel. The propane tanker flew a quarter of a mile and its impact dug a crater 10 feet deep.

Whitt, who was the first to respond to the fire, said crews have been successful in keeping the tanker cool since it caught fire Tuesday.

"Quite frankly, we are very lucky," he said. "We were really able to put a dent in the progression of the fire."

Extinguishing the fire near Sacramento would be welcome news to the thousands of residents forced to flee their homes. The American Red Cross said 270 people have taken shelter in three evacuation centers.

Leslie Reyes and her husband, a city maintenance worker, spent the night in their van outside the Lincoln Community Center, one of the three shelters set up for evacuees. Her oldest son slept outside next to the van while three other children were placed with family and friends.

She said they chose to stay outside because they were not allowed to bring in their 2 1/2-year-old toy rat terrier.

Reyes, 35, said she got a call at home Tuesday from her husband to evacuate. A short time later, an officer came to evacuate their street.

"At that point, I told my kids grab your memorables, pictures, things that are near and dear to your heart," Reyes said.

She said she appreciates the precaution despite the inconvenience. Reyes said she plans to relocate her family to her mother's in Antelope, about 15 miles away.

"Of course I would rather go home but it's better to be safe," Reyes said.

Roza Calderon, who lives with her family about a block away from the propane yard where the tanker was burning, described flames as high as utility lines before she evacuated.

"It was a big flame. It was getting worse," she said.

The 26-year-old accountant said she was staying with her husband, daughter and mother at a hotel in Sacramento.

At another shelter, the Kilaga Springs Lodge at a nearby Sun City community, volunteers set out 20 cots but had to add more as evacuees streamed in through the night.

One evacuee at the center, 21-year-old Richard Reyes, said he was hoping to be allowed back to his house soon.

"I guess we began to realize the situation was rapidly deteriorating when they had to call in a team from Texas. They called them super firefighters who fight petroleum fires," said the student and part-time mechanic.

Highway 65, a major commuter thoroughfare between Sacramento and Lincoln, remained closed near the blaze.